U.S. patent number 6,690,868 [Application Number 09/934,388] was granted by the patent office on 2004-02-10 for optical waveguide article including a fluorine-containing zone.
This patent grant is currently assigned to 3M Innovative Properties Company. Invention is credited to Mark T. Anderson, Alessandra O. Chiareli, Lawrence J. Donalds, James R. Onstott, Craig R. Schardt.
United States Patent |
6,690,868 |
Anderson , et al. |
February 10, 2004 |
**Please see images for:
( Certificate of Correction ) ** |
Optical waveguide article including a fluorine-containing zone
Abstract
An optical article including a core; at least one cladding
layer; and a narrow fluorine reservoir between the core and the
cladding layer. The fluorine reservoir has a higher concentration
of fluorine than either the cladding layer or the core. One
particular embodiment includes a core including a halide-doped
silicate glass that comprises approximately the following in
cation-plus-halide mole percent 0.25-5 mol % Al.sub.2 O.sub.3,
0.05-1.5 mol % La.sub.2 O.sub.3, 0.0005-0.75 mol % Er.sub.2
O.sub.3, 0.5-6 mol % F, 0-1 mol % Cl.
Inventors: |
Anderson; Mark T. (Woodbury,
MN), Schardt; Craig R. (Saint Paul, MN), Onstott; James
R. (Dresser, WI), Donalds; Lawrence J. (Mahtomedi,
MN), Chiareli; Alessandra O. (Saint Paul, MN) |
Assignee: |
3M Innovative Properties
Company (St. Paul, MN)
|
Family
ID: |
26968702 |
Appl.
No.: |
09/934,388 |
Filed: |
August 21, 2001 |
Current U.S.
Class: |
385/123 |
Current CPC
Class: |
C03B
37/01807 (20130101); C03C 3/06 (20130101); C03C
13/046 (20130101); C03C 23/0095 (20130101); C03C
25/104 (20130101); C03C 25/1065 (20130101); C03C
25/608 (20130101); G02B 6/02009 (20130101); G02B
6/03627 (20130101); G02B 6/03633 (20130101); G02B
6/0365 (20130101); G02B 6/03655 (20130101); G02B
6/03694 (20130101); G02B 6/2551 (20130101); H01S
3/06708 (20130101); C03B 2201/12 (20130101); C03B
2201/28 (20130101); C03B 2201/36 (20130101); C03B
2203/22 (20130101); C03C 2201/11 (20130101); C03C
2201/12 (20130101); C03C 2201/3417 (20130101); C03C
2201/3476 (20130101); C03C 2201/36 (20130101); C03C
2203/52 (20130101); G02B 6/02285 (20130101); H01S
3/06716 (20130101); H01S 3/06729 (20130101); H01S
3/06754 (20130101) |
Current International
Class: |
C03B
37/018 (20060101); C03C 23/00 (20060101); C03C
25/60 (20060101); C03C 13/00 (20060101); C03C
3/06 (20060101); C03C 25/10 (20060101); C03C
13/04 (20060101); G02B 6/036 (20060101); G02B
6/255 (20060101); G02B 6/02 (20060101); H01S
3/06 (20060101); H01S 3/067 (20060101); G02B
006/02 (); G02B 006/16 () |
Field of
Search: |
;385/123,124,125,126,127,128 |
References Cited
[Referenced By]
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Other References
Dianov, et al., "Application of Reduced-Pressure Plasma CVD
Technology to the Fabrication of Er-Doped Optical Fibers", 8397
Optical Materials, Aug. 3, 1994, No. 3, Amsterdam, NE, pp. 181-185.
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C. M. Miller, et al., "Optical Fiber Splices and Connectors: Theory
and Methods", Dekker, New York, Chapter 4, No Date. .
P.C. Becker, et al., "Amplifier Characterization and Design
Issues", Chapter 8, p. 264. No Date. .
H. Y. Tam, "Simple Fusion Splicing Technique for Reducing Splicing
Loss Between Standard Singlemode Fibres and Erbium-Doped Fibre",
Electronics Letters, Aug. 15.sup.th , 1991, vol. 27, No. 17, pp.
1597-1599. .
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Controlled Core Diffusion", Electronic Letters, No. 24, 1988, pp.
243-244. .
J. T. Krause, et al., "Splice Loss of Single-Mode Fiber as Related
to Fusion Time, Temperature, and Index Profile Alteration", IEEE
Journal of Lightwave Technology, LT-4, 1986, pp. 837-840. .
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of merit of 250ps/nm/dB", Electronics Letters, IIE Stevenage, GB,
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.
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-Doped Fiber for Optical Amplifier", Prodeedings of the European
Conference on Optical Communication (ECOC) Regular Papers; Berlin
(Sep. 27, 1992); vol. 1, Conference 18; pp. 505-508..
|
Primary Examiner: Nasri; Javaid H.
Attorney, Agent or Firm: Rosenblatt; Gregg H. Ball; Alan
Parent Case Text
RELATED CASES
The present case is related to co-pending, commonly-owned U.S.
Provisional Application No. 60/294,740, filed May 30, 2001,
entitled, "Optical Waveguide Article Including A
Fluorine-Containing Zone", and to co-pending, commonly-owned, U.S.
application Ser. No. 09/934,361, entitled "Method of Manufacture of
an Optical Waveguide Article Including a Fluorine-Containing Zone",
which was filed on the same day as the present application, both of
which are hereby incorporated by reference.
Claims
What is claimed is:
1. An optical article comprising: a) a core; b) at least one
cladding layer; c) a narrow fluorine reservoir between the core and
the cladding layer; d) wherein the fluorine reservoir has a higher
concentration of fluorine than the cladding layer and the core.
2. The optical article of claim 1, wherein the at least one
cladding layer, the core and the fluorine reservoir each has a
refractive index, wherein the refractive index of the fluorine
reservoir is less than the refractive index of the at least one
cladding layer and the refractive index of the core.
3. The optical article of claim 2, wherein the optical article
further comprises a substrate tube surrounding the cladding layer,
wherein the substrate tube has a refractive index that
substantially matches that of the cladding layer.
4. The optical article of claim 2, wherein the optical article
further comprises a substrate tube surrounding the cladding layer,
wherein the substrate tube has a refractive index that is greater
than that of the cladding layer.
5. The optical article of claim 1, wherein the cladding layer, the
core and the fluorine reservoir each has a refractive index,
wherein the refractive index of the fluorine reservoir
substantially matches the refractive index of the cladding layer,
and is less than that the refractive index of the core.
6. The optical article of claim 1, where the optical article is an
optical fiber and the core has a higher concentration of fluorine
at the edges of the core than the center of the core.
7. The optical article of claim 1, wherein the core comprises
silica, fluorine, and one or more rare earth ions.
8. The optical article of claim 7, wherein the rare earth ions are
chosen from Sc, Y, La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,
Yb, Lu.
9. The optical article of claim 7, wherein the core comprises at
least one additional modifier ion.
10. The optical article of claim 7, wherein the core comprises at
least one additional glass former ion.
11. The optical article of claim 7, wherein the core comprises at
least one additional glass former ion and one additional modifier
ion.
12. The optical article of claim 1 wherein the core comprises a
halide-doped silicate glass that comprises approximately the
following in cation-plus-halide mole percent: 1.5-3.5 mol %
Al.sub.2 O.sub.3, 0.25-1.0 mol % La.sub.2 O.sub.3, 0.0005-0.075 mol
% Er.sub.2 O.sub.3, 0.5-2 mol % F, 0-0.5 mol % Cl.
13. The optical article of claim 1, wherein the core comprises a
halide-doped silicate glass that comprises approximately the
following in cation-plus-halide mole percent0.25-5 mol% Al.sub.2
O.sub.3, 0.05-1.5 mol % La.sub.2 O.sub.3, 0.0005-0.75 mol %
Er.sub.2 O.sub.3, 0.5-6 mol % F, 0-1 mol % Cl.
14. The optical article of claim 1, wherein the core further
comprises a rare earth ion, lanthanum and aluminum.
15. The optical article of claim 1, wherein the core further
comprises a rare earth ion, germanium and aluminum.
16. The optical article of claim 1, wherein the core further
comprises a rare earth ion, germanium, aluminum, and lanthanum.
17. The optical article of claim 1, wherein the optical article is
an optical fiber preform.
18. An optical system including the optical article of claim 1.
19. The optical article of claim 17, wherein the preform has a DCRR
design.
20. The optical article of claim 17, wherein the preform has an
MCDR design.
21. The optical article of claim 17, wherein the preform has an
MCMR design.
22. The optical article of claim 17, wherein the preform has a DCLR
design.
23. The optical article of claim 17, wherein the preform has a DCDR
design.
24. The optical article of claim 17, wherein the preform has a MCRR
design.
25. An optical fiber comprising: a) a core; b) a concentric
cladding, the core and the cladding having a core/clad interface;
and c) a fluorine concentration zone extending from the core/clad
interface of the core and the cladding and overlapping across at
least a portion of the core and the cladding, the zone comprising a
highest concentration of fluorine approximately at the core/clad
interface.
26. An optical system including the fiber of claim 25.
27. The optical fiber of claim 25, the core having a center,
wherein the fluorine concentration near the center of the core is
higher than the fluorine concentration proximate the outer edge of
the cladding.
28. The optical fiber of claim 25, wherein the concentration of
fluorine in the cladding is higher than the concentration of
fluorine in the core.
29. The optical fiber of claim 25, wherein the zone does not
substantially impact the waveguiding properties of the fiber.
30. The optical fiber of claim 25, wherein the optical fiber is a
single mode, matched index cladding design.
31. The optical fiber of claim 25, wherein the cladding and the
zone each have an index of refraction and the indexes are
essentially matched.
32. The optical fiber of claim 25, wherein the optical fiber is a
rare earth doped fiber for optical signal amplification.
33. The optical fiber of claim 25, wherein the optical fiber is a
rare earth doped fiber.
Description
BACKGROUND OF THE INVENTION
The present invention relates to optical waveguide articles having
a novel optical design and to their manufacture. In particular, the
present invention relates to a novel optical fiber and preform
including a ring of high fluorine concentration and methods to
produce the article, and to core glass compositions.
The term optical waveguide article is meant to include optical
preforms (at any stage of production), optical fibers and other
optical waveguides. Optical fibers usually are manufactured by
first creating a glass preform. There are several methods to
prepare preforms, which include modified chemical vapor deposition
(MCVD), outside vapor deposition (OVD), and vapor axial deposition
(VAD). The glass preform comprises a silica tube. In MCVD different
layers of materials are deposited inside the tube; in OVD and VAD
different layers are deposited on the outside of a mandrel. The
resulting construction typically is then consolidated and collapsed
to form the preform, which resembles a glass rod. The arrangement
of layers in a preform generally mimics the desired arrangement of
layers in the end-fiber. The preform then is suspended in a tower
and heated to draw an extremely thin filament that becomes the
optical fiber.
An optical waveguide usually includes a light-transmitting core and
one or more claddings surrounding the core. The core and the
claddings generally are made of silica glass, doped by different
chemicals. The chemical composition of the different layers of an
optical waveguide article affects the light-guiding properties. For
certain applications, it has been found desirable to dope the core
and/or the claddings with rare earth materials. However, in rare
earth-doped silicates it is difficult to simultaneously achieve
high rare-earth ion solubility, good optical emission efficiency
(i.e. power conversion efficiency) and low background attenuation,
owing to the propensity for rare-earth ions to cluster in high
silica glasses.
Introduction of high concentrations of fluorine into the core glass
lowers the loss and improves rare earth solubility. Fluorine is
used in the core of optical fibers in which the fluorine diffuses
out of the core to raise the core index or to provide optical
coupling uniformity or mode field diameter conversion.
There are several methods to introduce fluorine into the core of an
optical fiber: (1) chemical vapor deposition (CVD), which includes
modified chemical vapor deposition (MCVD), outside vapor deposition
(OVD), vapor axial deposition (VAD), and surface plasma chemical
vapor deposition (SPCVD); (2) solution doping CVD-derived soot with
fluoride particles or doping with a cation solution and then
providing a source of fluoride (gas or HF solution); (3) sol-gel
deposition of a fluoride containing core layer; (4) direct melting
techniques with fluoride salts; and (5) gas phase diffusion of
fluorine into the core layer before or during collapse.
Each method has drawbacks. For example, method (1), direct
incorporation of fluorine by CVD methods, currently is limited to
about <2 wt % fluorine unless plasma CVD is used. Deposition
conditions generally must be reengineered every time the relative
amount of fluorine is changed. In a solution doping embodiment,
soot porosity along with the doping solution concentration
determine the final glass composition. Constant re-engineering is
especially problematic for solution doping where the melting point
and viscosity of the glass, and thus soot porosity change rapidly
with fluorine concentration.
In method (2), solution doping with fluoride particles may lead to
inhomogeneities from particles settling out of solution during the
contact period. Exposure of a cation-doped soot to a fluoride
containing solution can lead to partial removal of cations owing to
resolubilization in the fluoride containing liquid. In the case
that a gas is used as a fluoride source, the gas may etch the
porous soot and alter the silica to metal ion ratio.
For method (3), sol-gel deposition, drawbacks include the
propensity of sol-gel derived layers to crack and flake. If thin
layers are used to attempt to avoid these problems, the need arises
for multiple coating and drying passes.
For (4), direct melting techniques, drawbacks include the handling
of hygroscopic metal salts, many of which present a contact hazard.
In addition, there are difficulties uniformly coating a melt on the
inside of a tube.
Finally, for method (5), gas phase reactions, the gas may etch some
of the silica and change the silica to dopant ion
concentration.
Fluorine (in the form of fluoride ions) has a high diffusion
coefficient in oxide glasses. Fluorine will rapidly diffuse from a
region of higher concentration to lower concentration. The ability
of fluorine to rapidly diffuse is utilized to mode match fibers of
dissimilar physical core dimensions. Fluorine diffusion out of the
core into the cladding layer is used in the production of fiber
optic couplers and splitters to improve the uniformity of optical
coupling. Fluorine diffusion out of the core also may be used for
mode field diameter conversion fiber.
Direct fluorination of the core of a fiber to provide a graded
coefficient of thermal expansion (CTE) and viscosity may be
beneficial to the optical properties, such as a reduction in the
stimulated Brillion scattering.
Also, it is further recognized that the presence of large amounts
of fluoride in oxyfluoride glasses is beneficial to prevent phase
separation and clustering of rare earth, and also that clustering
of fluorescing rare earth ions, such as Er.sup.3+, has deleterious
effects on spectral breadth, excited-state lifetimes, amplification
threshold (pump power needed to invert an optical amplifier), and
power conversion efficiency of an optical amplifier.
Rare-earth-doped aluminosilicate glasses have been doped with
fluorine. For example, it has been reported that rare-earth-doped
aluminosilicate glass doped with fluorine exhibits remarkable light
emission characteristics, including high-gain amplification and
broad spectral width.
Fluorine also may be doped into the cladding of optical fiber
preforms. Depressed index claddings can, for example, suppress
leaky mode losses in single mode fibers. Depressed index clad
designs, where the index lowering dopant ions, such as F and B, are
in the cladding have been used to control chromatic dispersion, for
example.
Preforms may be made from fluorine-containing substrate tubes. Such
tubes may be used to form silica core waveguides by diffusion of
index lowering species, such as fluorine, out of the inner portion
of the tube prior to collapse. In depressed index substrate tubes,
there is fluorine in the substrate tube to provide favorable
waveguiding properties or to diffuse out of the tube entirely to
raise the local index of the innermost region.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a depiction of the refractive index profile and a
corresponding schematic cross-section of a first embodiment of an
optical waveguide article having a matched-clad depressed-ring
(MCDR) design in accordance with the present invention.
FIG. 2 is a depiction of the refractive index profile and a
corresponding schematic cross-section of a second embodiment of an
optical waveguide article having a matched-clad matched-ring (MCMR)
design in accordance with the present invention.
FIG. 3 is a depiction of the refractive index profile and a
corresponding schematic cross-section of a third embodiment of an
optical waveguide article having a depressed-clad lower-ring (DCLR)
design in accordance with the present invention.
FIG. 4 is a depiction of the refractive index profile and a
corresponding schematic cross-section of a fourth embodiment of an
optical waveguide article having a depressed-clad depressed-ring
(DCDR) design in accordance with the present invention.
FIG. 5 is a depiction of the refractive index profile and a
corresponding schematic cross-section of a fifth embodiment of an
optical waveguide article having a matched-clad raised-ring (MCRR)
design in accordance with the present invention.
FIG. 6 is a depiction of the refractive index profile and a
corresponding schematic cross-section of a sixth embodiment of an
optical waveguide article having a depressed-clad raised-ring
(DCRR) design in accordance with the present invention.
FIG. 7 is a depiction of the schematic cross-section of a seventh
embodiment of an optical waveguide article having a barrier layer
design in accordance with the present invention.
FIG. 8 is a depiction of the schematic cross-section of an eighth
embodiment of an optical waveguide article having a double barrier
layer design in accordance with the present invention.
FIG. 9 is a graph of fluorine concentration vs. radial position
starting from the center of the core for a preform with an initial
uniform fluorine concentration in the core.
FIG. 10 is a graph of fluorine concentration vs. radial position
starting from the center of the core for a preform having a
fluorine high concentration ring as described in the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates the refractive index profile depiction and
schematic cross-section of a first embodiment of an optical
waveguide article 100 in accordance with the present invention.
FIGS. 2-6 similarly illustrate the refractive index profile and
cross-section of a second, third, fourth, fifth, and sixth
embodiment, respectively, of the present invention. Similar
elements are identified using reference numerals having the same
last two digits. The axes of the refractive index profile
depictions for FIGS. 1-6 are distance from center (r) vs.
refractive index (n). The axes are unitless and the n-axis is not
necessarily intersected at the zero point by the r axis, because
the purpose of the Figures is to illustrate the profile shapes and
index relations rather than profiles for specific optical articles.
Please note that the drawings are for illustrative purposes only,
and are not necessarily meant to be to scale. Those skilled in the
art will readily appreciate a variety of other designs that are
encompassed by the present invention.
The term optical waveguide article is meant to include optical
preforms (at any stage of production), optical fibers, and other
optical waveguides. FIG. 1 includes a depiction of the refractive
index profile 102 and a corresponding schematic cross-section of a
first embodiment of an optical waveguide article 100 having a
matched-clad depressed-ring (MCDR) design in accordance with the
present invention. The article 100 includes a core 110 having a
radius r.sub.1, a fluorine-containing zone or ring 120 having a
radius r.sub.2 surrounding and concentric with the core, one or
more cladding layers 130 having a radius r.sub.3 adjacent to the
ring 120 and concentric with the core, and a substrate tube 140
surrounding the cladding layer 130. The cladding 130 is a layer of
high purity glass concentric with the core 110. The cladding 130
may be circular, oval, square, rectangular, or other shapes in
cross-section. In an optical preform, the substrate tube 140 is a
high-silica tube, which is hollow before formation of the inner
layers and collapse. The base component of the core 110, the zone
120, and the cladding layers 130 generally also is silica, doped
with different chemicals for desired optical characteristics. In
alternative embodiments, the cladding layer 130 may include more
than one cladding layer.
As explained in more detail in the method of manufacture discussion
below, optical fibers are drawn from the optical preforms. The
optical fibers maintain the core and cladding arrangement of the
preform. Therefore, FIGS. 1-6 also may illustrate the
cross-sectional index profile for an optical fiber resulting from a
similar optical preform. However, the fluorine zone generally
diffuses into the core and/or the cladding, creating a fluorine
"zone" rather than a reservoir. In the present and following
embodiments, it must be understood that when the fluorine has been
diffused, the fluorine concentration zone will be functionally
either part of the cladding or core with respect to optical
performance.
When the optical article is a preform, the fluorine containing zone
120 acts as a "reservoir" outside of the core from which fluorine
may be diffused into the core in subsequent processing steps. The
concentration of fluorine in the zone 120 is greater than that in
the innermost cladding 130 and the core 110. Optionally, the zone
120 also has an index similar to that of the cladding. In the
present invention, the zone 120 allows net diffusion of fluorine
into the core from the surrounding glass, not diffusion from the
core to the surrounding glass.
The zone 120 also is "optically narrow". The term optically narrow
is defined such that the fluorine-ring differential width (outer
radius of fluorine ring minus the inner radius of the fluorine
ring) is approximately less than 1/4 the core diameter and that the
presence of the fluorine ring does not significantly negatively
impact the waveguiding properties of the final fiber. The inventive
article is intended to have optical properties substantially
identical to an article of similar design without the fluorine
ring, referred to as the standard. Having a similar design is
defined as occurring when the difference in the .DELTA. (.DELTA. is
the core refractive index minus the refractive index of silica) of
the cores of the fibers are less than 5%; the difference in the
.DELTA. of the claddings is less than 5%, the core diameters are
within 2%, and the cladding diameters (minus the fluorine-ring
differential width in the fluorine-ring case) are within 2%.
Negative impact is defined as not being able to simultaneously meet
the following specifications in the present inventive fiber as
compared to a standard fiber of similar design without the fluorine
reservoir: the fundamental mode can propagate at operating
wavelength, mode field diameter is 4.5 to 6 microns, background
loss at operating wavelength <15 dB/km, and the (second mode)
cutoff is less than the amplifier pump wavelength (e.g. for erbium
this is either 850-950 nm or <1480 nm, depending on the pump
wavelength used for the amplifier).
The present invention includes a method to manufacture optical
fiber having a low loss and a uniform distribution of rare earth
ions. Such fiber is particularly useful in optical amplification
applications, especially in dense wavelength division multiplexing
(DWDM) systems.
Introduction of fluorine into aluminosilicates or
germano-aluminosilicates provides high gain, wider bandwidth, and
ease of splicing to silica glasses. The present invention offers
designs with high total rare-earth ion concentrations (e.g. La+Er)
in which surprisingly low concentrations of fluorine
(>.about.0.15 wt % (>0.5 mol %)) can provide high rare earth
solubility and low background attenuation. Additionally, in a
solution-doping/MCVD approach, direct fluorination of the core
requires re-engineering the soot deposition and solution doping
processes. Thus, the invention provides unexpectedly low-loss
rare-earth-doped glass in a manufacturing process compatible with
standard solution-doping/MCVD.
In addition, except in the infinite time/temperature limit, direct
fluorination of the core gives a different fluorine concentration
profile across the fiber than a fluorine ring design. It appears to
be quite advantageous to optical properties (esp. loss) and
fusability to have a high concentration of fluorine in the core and
in the zone between core and cladding. This is a major difference
between the present fluorine ring approach and methods (2)-(5)
listed above (i.e. solution doping, sol-gel, direct melting, or gas
phase reactions during collapse).
An advantage of the present invention over preparing, for example,
erbium-doped oxide fiber with no fluorine reservoir, is a reduction
of >.about.3 dB/km in background loss measured at 1200 nm. In an
MCVD/solution doping manufacturing process, one major advantage of
a fluorine reservoir approach over direct fluorination of the core
is that the silica soot does not have to be re-engineered to
contain fluorine.
A fiber in accordance with the present invention is readily
spliceable and may be prepared with desirable fundamental mode
cutoff, acceptable dispersion and mode field diameter, and low
polarization mode dispersion. The method and article of the present
invention also provide lower viscosity of the glass proximate to
the core, and allow lower background attenuation than in
depressed-well erbium-doped fiber without a fluorine ring. The
invention also provides a method to tailor the fluorine
distribution radially. As the diffusion rate of fluorine ions is
much greater than that of the rare earth ions, the invention also
allows embodiments having a non-equilibrium distribution of rare
earth ions in an oxyfluoride glass (i.e. rare-earth-rich regions
that can be fluorinated) that would not form from a homogeneous
oxyfluoride melt. This can lead to a wider variety of rare earth
ion sites in the glass, which contributes to a broader gain
spectrum. Broader gain spectra are highly advantageous for DWDM
optical amplifiers.
Referring back to FIG. 1, the zone 120 includes glass of high
fluorine content proximate to the core 110. The fluorine
concentration in the zone 120 is greater than the fluorine
concentration in either the core 110 or the cladding 130.
Concentration may be measured in mol percent using wavelength
dispersive X-ray analysis (WDX) or secondary ion mass spectrometry
(SIMS). The zone 120 also is generally narrower than either the
core 110 or the cladding 130, and it is designed not to interfere
with the optical functioning of either the core 110 or the cladding
130.
In an embodiment of the optical article of FIG. 1, the optical
article 100 is single mode optical preform and has a matched-index
cladding design (r.sub.3) with a thin depressed-index (d.sub.1)
high-fluorine-content ring (r.sub.2) around the core (r.sub.1).
d.sub.1 is the index profile difference between the ring 120 and
the cladding 130. It is intended generally that the fluorine ring
(reservoir) not substantially impact the waveguiding properties of
the fiber. For example, the fundamental mode cutoff still allows
single-mode operation in the 1500-1650 nm region and the dispersion
profile of the fiber is not significantly changed relative to a
control fiber without the fluorine reservoir region.
The zone of high fluorine concentration 120 has a different
chemical composition than the cladding 130. However, the reservoir
region 120 will still interact with transmitted light and will
serve optically as part of the cladding 130, especially in the
final fiber after fluorine diffusion has occurred.
In one specific version of the embodiment illustrated in FIG. 1,
the fiber has these properties: (1) NA is >0.2, preferably
>0.25, (2) the mode field diameter is <6 .mu.m, preferably
<5.5 .mu.m, (3) background attenuation measured at 1200 nm is
<20 dB/km, preferably <15 dB/km, more preferably <10
dB/km, (4) fundamental mode cutoff is greater than 1800 nm (5)
second mode cutoff is <1480 nm, preferably <980 nm. These
same fiber specifications also may be used in embodiments of the
designs in FIGS. 2-8.
FIG. 2 is a depiction of the refractive index profile 202 and a
corresponding schematic cross-section of a second embodiment of an
optical waveguide article 200 having a matched-clad matched-ring
(MCMR) design in accordance with the present invention. In an
exemplary embodiment, the optical article 200 is a single mode
optical preform and has a matched-index cladding 230 (r.sub.3) with
a thin matched-index high-fluorine-content ring 220 (r.sub.2)
around the core 210 (r.sub.1).
FIG. 3 is a depiction of the refractive index profile 302 and a
corresponding schematic cross-section of a third embodiment of an
optical waveguide article 300 having a depressed-clad lower-ring
(DCLR) design in accordance with the present invention. In an
exemplary embodiment, the article 300 is single mode optical
preform and has a depressed-index (d.sub.1) inner cladding 330
(r.sub.3) and outer cladding 350 design with a thin
further-depressed-index (d.sub.2) high-fluorine-content ring 320
(r.sub.2) around the core 310 (r.sub.1). d.sub.1 is the "well
depth", that is, index difference of the depressed index for the
inner cladding with respect to the outer cladding. d.sub.2 is the
index difference of the refractive index for the ring with respect
to the outer cladding. FIG. 4 is a depiction of the refractive
index profile 402 and a corresponding schematic cross-section of a
fourth embodiment of an optical waveguide article 400 having a
depressed-clad depressed-ring (DCDR) design in accordance with the
present invention. In an exemplary embodiment, the article 400 is
single mode optical fiber and has a depressed-index inner cladding
430 and matched-index outer cladding 450 design (r.sub.3) with a
thin depressed-index (d.sub.2) high-fluorine-content ring 420
(r.sub.2) around the core 410 (r.sub.1).
FIG. 5 is a depiction of the refractive index profile 502 and a
corresponding schematic cross-section of a fifth embodiment of an
optical waveguide article 500 having a matched-clad raised-ring
(MCRR) design in accordance with the present invention. The present
exemplary article 500 is single mode optical preform and has a
matched-index cladding 530 design (r3) with a thin raised-index
high-fluorine-content ring 520 (r2) approximately at the core
510/clad 530 interface (r1). The core/clad interface 515 is defined
as the radial position where the measured refractive index equals
the average of the equivalent step index (ESI) core and ESI clad
values.
FIG. 6 is a depiction of the refractive index profile 602 and a
corresponding schematic cross-section of an sixth embodiment of an
optical waveguide article 600 having a depressed-clad raised-ring
(DCRR) design in accordance with the present invention. The
exemplary article 600 is single mode optical preform and has a
depressed-index inner cladding 630 and matched-index outer cladding
650 (r.sub.3) with a thin raised-index (d.sub.1)
high-fluorine-content ring 620 (r.sub.2) approximately at the
core/clad interface 610 (r.sub.1). The refractive index of the
depressed clad 630 and the fluorine ring 620 are essentially
matched.
In yet another embodiment of an optical preform 700, illustrated in
FIG. 7, a diffusion barrier 760, such as a high silica ring, is
placed at a distance greater from a core 710 than the proximate
fluorine ring 720. The diffusion barrier layer 760 is generally
high silica or other material that decreases the diffusion rate of
fluorine compared to the diffusion rate of fluorine in the cladding
layers. Its purpose is to reduce the diffusion of fluorine into the
cladding 730 thereby allowing more of the fluorine in the reservoir
720 to eventually diffuse into the core 710. The diffusion barrier
760 does not substantially impact the waveguiding properties of the
fiber.
In contrast with references in which barrier layers have been
incorporated into optical fibers to prevent diffusion of
loss-raising impurities into regions near the core, the present
embodiment uses barrier layers to prevent diffusion of fluorine out
of the region near the core, and enhance the amount of fluorine in
the core. The diffusion barrier 760 decreases the diffusion of
fluorine away from the core and allows more of it to eventually
diffuse into the core.
The use of barrier layer and the reservoir concept of the present
invention, allows for the crafting of novel embodiments having
fluorine diffusion regions. In an alternative embodiment 800,
illustrated in FIG. 8, a first barrier layer 860 may be placed in
or near the core region 810, exemplarily near the boundary with a
zone of high-fluorine concentration 820. The first barrier layer
860 decreases the rate of diffusion of fluorine into the inner
portions of the core 810. A second barrier layer 862 may be placed
in or near the cladding region 830 to decrease the rate of
diffusion of fluorine across the outer portions of the cladding or
between cladding layers.
Referring to the embodiments illustrated in FIGS. 1-8, the present
invention is particularly useful for forming optical articles
having fluorosilicate core glasses. Active rare-earth-doped
compositions that contain passive-rare-earths in a
fluoroaluminosilicate or fluoroaluminogermanosilicate host with the
concentrations of fluorine achievable in our invention are believed
to be novel. In one embodiment, the core glass is a fluorosilicate
that contains rare earth ions. More preferably, the core glass is a
fluorosilicate that contains one or more active rare earth ions. An
active rare earth ion is defined as one that exhibits a useful
fluoresce in the near infrared (e.g. Yb3+, Nd3+, Pr3+, Tm3+, and/or
Er3+). In other embodiments, the fluorosilicate glass contains
additional glass forming dopants (e.g. Al, Ge, Sb, and/or Sn) and
one or more active rare earth ions. In another embodiment the
fluorosilicate glass contains additional glass modifier ions (e.g.
Na, Ca, Ti, Zr, and/or rare earths) and one or more active rare
earth ions.
One particular optical article according to the present invention
includes a core and a concentric cladding in which the core
comprises a halide-doped silicate glass that comprises
approximately the following in cation-plus-halide mole percent:
85-99 mol % SiO.sub.2, 0.25-5 mol % Al.sub.2 O.sub.3, 0.05-1.5 mol
% La.sub.2 O.sub.3, 0.0005-0.75 mol % Er.sub.2 O.sub.3, 0.5-6 mol
%, F, 0-1 mol % Cl. In another embodiment the glass comprises:
93-98 mol % SiO.sub.2, 1.5-3.5 mol % Al.sub.2 O.sub.3, 0.25-1.0 mol
% La.sub.2 O.sub.3, 0.0005-0.075 mol % Er.sub.2 O.sub.3, 0.5-2 mol
% F, 0-0.5 mol % Cl.
The term cation-plus-halide mole percent (hereafter simply mol %)
is defined as: 100 times the number of specified atoms divided by
the total number of non-oxygen atoms, as determined by wavelength
dispersive X-ray analysis or other suitable technique. For example,
to determine the relative amount of silicon atoms in the oxyhalide
glass one would divide the number of silicon atoms by the number of
silicon plus aluminum plus lanthanum plus erbium plus flourine plus
chlorine atoms and multiply the result by 100. To avoid any
ambiguity we state the first above compositional ranges in
approximate weight percent also: 78.2-99.1 wt % SiO.sub.2, 0.4-7.7
wt % Al.sub.2 O.sub.3, 0.3-7.4 wt % La.sub.2 O.sub.3, 0.003-4.35 wt
% Er.sub.2 O.sub.3, 0.16-1.7 wt % F, 0-5 wt % Cl. The glass
contains oxygen in the requisite amount to maintain charge
neutrality. The glass may additionally contain small amounts of
hydrogen, for example less than 1 ppm, predominantly in the form of
hydroxyl ions and may further contain small amounts of other
elements from source materials, in the form of ions or neutral
species, for example in concentrations less than 100 ppb.
In yet another embodiment, the fluorosilicate glass contains glass
forming dopants and glass modifier ions and an active rare earth
ion (e.g. Yb3+, Nd3+, Pr3+, Tm3+, and/or Er3+). In other
embodiments, the fluorosilicate glass may contain non-active rare
earth modifier ions (e.g. La, Lu, Y, Sc, Gd, or Ce), active rare
earth ions, and germanium. In another embodiment the fluorosilicate
glass contains non-active rare earth modifier ions, active rare
earth ions, and aluminum. The fluorosilicate glass also may contain
aluminum, lanthanum, and erbium.
In a specific embodiment used for optical amplification, the core
comprises a halide-doped silicate glass that comprises
approximately 1.5-3.5 mol % Al.sub.2 O.sub.3, 0.25-1 mol % La.sub.2
O.sub.3, 5-750 ppm Er.sub.2 O.sub.3, 0.5-6.0 mol % F, and 0-0.5 mol
% Cl. One particular exemplary embodiment also may further include
0-15 mol % GeO.sub.2. In another particular embodiment, the core
comprises silicate (SiO2) glass including approximately the
following in cation-plus-halide mole percent: 1.5-3.5% Al.sub.2
O.sub.3, 0.25-1.0% La.sub.2 O.sub.3, 5-750 ppm Er.sub.2 O.sub.3,
0.5-2.0% F, 0-0.5% Cl.
Erbium-doped SiO.sub.2 --Al.sub.2 O.sub.3 ; SiO.sub.2 --Al.sub.2
O.sub.3 --La.sub.2 O.sub.3 ; SiO.sub.2 --Al.sub.2 O.sub.3
--GeO.sub.2 ; and SiO.sub.2 --Al.sub.2 O.sub.3 --La.sub.2 O.sub.3
--GeO.sub.2 glasses are useful in optical amplification.
Oxyfluoride compositions of the first type that contain a high
concentration of fluorine (e.g. at least 2 wt %), as made by SPCVD,
for example, provide broad Er.sup.3+ emission spectra, and low
attenuation. Optical amplifier fibers in accordance with the
present invention show unexpected benefits in lanthanum
aluminosilicate type glasses from the incorporation of relatively
low concentrations of fluorine >0.5 mol % (.about.0.15 wt %) in
the core, namely, a reduction in background attenuation with
retention of small mode field diameter, fundamental mode cutoff
less than 980 .mu.m, and spliceability to other optical fibers.
Since the diffusion rates of fluoride are much greater than those
of the rare earth ions, optical fibers in accordance with the
present invention allow a non-equilibrium distribution of rare
earth ions in an oxyfluoride glass (i.e. erbium and fluorine rich
domains) that would not form from a homogeneous oxyfluoride melt.
This may lead to a wider variety of rare earth ion sites in the
glass, which contributes to a broader gain spectrum, highly
advantageous for DWDM optical amplifiers.
Method of manufacture
The present invention further relates to methods of manufacture of
an optical waveguide article, including methods to introduce
fluorine into the core of the optical fiber by diffusion to modify
optical and physical properties of the fiber. More specifically the
invention discloses methods to deposit a high concentration of
fluorine-containing glass in a region proximate to the core in a
fiber preform.
To manufacture an optical waveguide article in accordance with the
present invention, a substrate tube, such as tubes 140, 240, 340,
440, 540 and 640, is first provided. The substrate tube generally
is a hollow synthetic silica rod, such as those available from
General Electric, USA. The tube is cleaned, such as by an acid
wash, to remove any foreign matter and is mounted in a lathe for
deposition of the inner layers.
The methods to deposit the inner layers are well known, such as
MCVD, sol-gel, glass melting and coating. One or more cladding
layers are formed. In a particular embodiment, the tube was placed
on a CVD lathe. One or more clearing passes may be made to clean
and etch the inside of the tube. Gasses were delivered to the
inside of the glass tube. A torch, such as a hydrogen/oxygen torch,
was traversed along a length of the tube during the clear pass.
Flow rates of the gases, flame temperature, and carriage speeds for
the torch are computer controlled in accordance with the desired
chemical compositions for the manufactured product.
Certain embodiments, such as those illustrated in FIGS. 3 and 4,
include an outer cladding layer and an inner cladding layer.
Following the clearing pass, the outer cladding is deposited by
modified chemical vapor deposition (MCVD). In this process porous
glass is deposited on the inner walls of the substrate tube
downstream of the burner by thermophoresis. The burner consolidates
the deposited glass in the center of the flame. The inner cladding
is deposited using a number of passes. The refractive index of the
cladding layers may be controlled by the chemical composition in
each pass. In one particular embodiment, the innermost cladding
comprises 98.5 mol % silica, 0.8 mol % fluorine and 0.7 mol %
phosphorus oxide (as PO.sub.2.5 throughout).
The fluorine ring is applied using one or more passes of the torch
while introducing the desired higher concentration of fluorine. The
fluorine reservoir region also may contain relatively high contents
of index raising dopant (e.g. P) to maintain a matched index.
Methods to deposit the fluorine reservoir include, but are not
limited to, MCVD, plasma enhanced CVD (PECVD), sol-gel doping, and
coating the tube with a melted fluoride glass.
The chemical materials and the concentration of these materials in
the reservoir are tailored for different applications and for
different desired zones of diffusion. The concentration of fluorine
in the core and the cladding also may affect the desired
concentration of fluorine in the reservoir. For example, a
fluorinated cladding would increase the net inward diffusion of
fluorine from the reservoir into the core, by keeping the fluorine
concentration in the reservoir high longer. Some fluorine diffusing
out into the cladding would be replaced by fluorine diffusing into
the reservoir from the cladding (the concentration gradient would
be less steep on the outside of the reservoir than on the inside,
so the net diffusion rate would be lower on the outside of the
reservoir than on the inside.) Additionally, one could also add a
diffusion enhancer such as phosphorus oxide to the core region
inside the fluorine reservoir, to create a preferential inward
diffusion of fluorine.
Fluorine concentration is determined by the relative flows of
fluorine precursor vs. other components. In an exemplary
embodiment, the fluorine concentration in the fluorine reservoir is
at least 30% higher than the fluorine concentration in either the
core or the innermost cladding layer. In another design, the
fluorine concentration in the fluorine reservoir is at least 50%
higher than the fluorine concentration in either the core or the
innermost cladding layer. Finally, in yet another design, the
fluorine concentration in the fluorine reservoir is at least 100%
higher than the fluorine concentration in either the core or the
innermost cladding layer.
Some exemplary embodiments include fluorine concentrations in the
fluorine reservoir of between at least 0.7 mol % to at least 4.0
mol %. Other exemplary embodiments include even higher fluorine
concentrations ranging from greater than 80 mol % silica and less
than 20 mol % fluorine, to less than 5 mol % fluorine.
The fluorine reservoir also may comprise phosphorus oxide. The
concentration of phosphorus oxide may be approximately equal to,
less than, or greater than the concentration of fluorine. One
exemplary embodiment includes between less than 1% phosphorus oxide
to less than 20% phosphorus oxide. In another exemplary matched
index embodiment, the reservoir comprises about 95.7-99.7 mol %
silica, about 0.3-4 mol % fluorine and about 0-0.3 mol % phosphorus
oxide.
The core may be formed by a variety of methods, including MCVD,
solution doping, sol-gel doping, or PECVD.
In various embodiments, the core comprises silica, an active rare
earth dopant, and at least one additional component. The additional
components may include F and Cl. The additional components of the
core also may comprise one or more glass formers or conditional
glass formers, such as Ge, P, B, Cl, Al, Ga, Ge, Bi, Se, and Te.
The additional components also may comprise one or more modifiers,
such as Zr, Ti, rare earths, alkali metals, and alkaline earth
metals.
The active rare earth dopant may include rare earth ions that
fluoresce in the near infrared (e.g. Yb3+, Nd3+, Pr3+, Tm3+, or
Er3+). In addition to the active rare earth dopant, the core also
may include one or more of La, Al, and Ge. In one particular
embodiment, the Al is less than 10 mol %. In an even more
particular exemplary embodiment, the Al concentration is less than
7 mol %. In a particular embodiment, the dopant includes La, in
which La is less than 3.5 mol %. In a particular embodiment, the
dopant includes Ge, in which Ge is less than 25 mol %.
The core also may include one or more non-active rare earth ions
(RE), such as La, Y, Lu, Sc. In one embodiment, the non-active rare
earth concentration is less than 5 mol %. In particular
embodiments, the composition of the core has molar composition of:
SiO.sub.2 75-99%, Al.sub.2 O.sub.3 0-10%, RE.sub.2 O.sub.3
0-5%.
After deposition of the core, the tube was then consolidated and
collapsed into a seed preform.
In one embodiment subsequent thermal processing is performed to
adjust the core-to-clad ratio to achieve a desired core diameter in
the final fiber. Such subsequent processing may involve multiple
stretch and overcollapse steps. The completed preform may then be
drawn into an optical fiber. In a particular embodiment, the
preform was hung in a draw tower. The draw tower included a torch
or furnace to melt the preform, and a number of processing
stations, such as for coating, curing and annealing.
The prepared preform is processed, such as by heating, such that a
portion of the fluorine in the proximate high fluorine
concentration layer diffuses into the core and/or the cladding. The
fluorine may diffuse out of the reservoir during collapse, during
heat-treatment of the preform, during the stretch/overcollapse
process, during the draw of the resulting optical fiber, and/or,
during a post-treatment of the fiber as an independent step. While
diffusing fluorine from, for example, the core to the cladding, has
been previously discussed, it is believed that the present
invention offers a novel method to diffuse fluorine from a
reservoir into the core and/or the cladding before, during, or
after draw to reduce loss and improve dopant ion distribution in
rare-earth-doped fibers.
Thermal processing of the preform, other than that described above,
such as isothermal heating in a tube furnace may be used to further
enhance the fluorine content in the core of the fiber or to modify
the radial distribution of fluorine. Different chemicals, such as F
and P, in the reservoir will diffuse at different rates, so
components may form distinct "concentration zones".
The graphs in FIGS. 9 and 10 show fluorine concentration as a
function of distance from the core for an optical article, a
preform or an optical fiber, which has been processed to diffuse
fluorine from the fluorine reservoir. The resulting optical article
includes a core and a concentric cladding. The core and the
cladding are proximate to each other and have a core/clad
interface, as defined above. A fluorine concentration zone overlaps
at least a portion of the core and the cladding. When the fluorine
has been diffused, the physical distribution of the fluorine
concentration zone will be, from an optical functionally
perspective, part of the cladding and/or the core.
FIG. 9 is a graph of fluorine concentration for differing values of
the diffusion time-diffusivity product vs. radial position starting
from the center of the core for a preform with an initial uniform
fluorine concentration in the core (no fluorine in the cladding).
The curves represent concentration profiles for different values of
the diffusivity-diffusion time product: (1) Dt=0.001, (2) Dt=0.01,
(3) Dt=0.1, (4) Dt=1. In the directly fluorinated case, FIG. 9,
(uniformly distributed core dopant), the maximum concentration of
fluorine is always at the center of the core.
FIG. 10 is a graph of fluorine concentration for differing values
of the diffusion time-diffusivity product vs. radial position
starting from the center of the core for a preform having a
fluorine high concentration ring as described in the present
invention. Again, the curves represent concentration profiles for
different values of the diffusivity-diffusion time product: (1)
Dt=0.001, (2) Dt=0.01, (3) Dt=0.1, (4) Dt=1. In the fluorine
reservoir diffusion design of FIG. 10, the maximum concentration
can be tailored from the core/clad interface to the center of the
core. This allows a large degree of flexibility in draw conditions
and final stress states of the fiber.
The fluorine reservoir in a pre-treated preform according to the
present invention is generally placed at the core/clad interface.
Accordingly, in most cases, the highest concentration of fluorine
for the diffusion treated optical article will be at the interface.
However, as illustrated in FIGS. 9 and 10, as the diffusion time
increases the distribution of fluorine becomes more normalized.
Accordingly, there may be embodiments of treated optical articles
in which the fluorine concentration is more evenly distributed
across the core and/or the cladding. Alternatively, one may take
advantage of the concentric geometry of the core and use the
overlap of radial diffusion gradients to create zones of higher
fluorine concentration at or proximate the center of the core.
Similarly, the speed of diffusion may be different within the core
and the cladding, depending on the doping and materials of the
different regions, as well as the diffusion treatment steps.
Moreover, diffusion barriers may be placed within the core and the
cladding to tailor the radial concentration distribution of
fluorine.
Using the different tools described by the present invention, a
large variety of fluorine concentration profiles may be achieved.
In one particular embodiment, the fluorine concentration near the
center of the core is higher than the fluorine concentration at the
outer edge of the cladding. In another embodiment, the reverse is
true, having a higher concentration of fluorine in the cladding
than in the center of the core.
EXAMPLES
The present invention may be better understood in light of the
following examples:
Example 1
Control
A preform with a depressed index inner clad was fabricated by MCVD
techniques. Five deposition passes with SiF.sub.4 (flow rates of 30
sccm), POCl.sub.3 (100 sccm), and SiCl.sub.4 (950 sccm) were made
to prepare the inner cladding. The core was erbium-doped lanthanum
aluminosilicate. The collapsed preform was sectioned, stretched,
and overcollapsed for draw. Fiber was drawn from this preform and
measurements were made of the mode field diameter, cutoff
wavelength, and loss at 1200 nm. Wavelength dispersive X-ray
analysis of the preform drop yielded .about.0.3 mol % fluorine in
the core and 2.1 mol % fluorine and <0.3 mol % phosphorous in
the depressed index inner cladding layer.
Example 2
Fluorine Reservoir
A DCLR preform, having a profile similar to that illustrated in
FIG. 3, was fabricated by MCVD techniques. Five deposition passes
with SiF.sub.4 (30 sccm), POCl.sub.3 (100 sccm), and SiCl.sub.4
(950 sccm) were made to prepare the inner cladding, and a sixth
deposition pass with SiF.sub.4 (flow rates of 350 sccm), POCl.sub.3
(100 sccm), and SiCl.sub.4 (350 sccm) was made to yield a
fluorosilicate reservoir region with 4 mol % fluorine. The core was
erbium-doped lanthanum aluminosilicate. The collapsed preform was
sectioned, stretched, and overcollapsed for draw. The fiber was
drawn and characterized in the same manner as in Example 1.
Wavelength dispersive X-ray analysis of the preform drop yielded a
core with >0.5 mol % (>0.15 wt %) fluorine in the core, a
fluorine ring with .about.4 mol % fluorine, and an inner cladding
with .about.2.1 mol % fluorine.
TABLE 1 Comparison of Fibers in Examples 1 and 2 Fcore (fluorine
Fring (fluorine in the core of in the ring of Mfd (mode the preform
the preform field diameter Bkgd. Loss Fiber type drop) drop) of
fiber) Cutoff at 1200 nm Control .about.0.3 mol % N.A. 5.1 .mu.m
890 nm 10.0 dB/km DCLR >0.5 mol % .about.4 mol % 5.3 .mu.m 920
nm 7.0 dB/km
The gain shape of the DCLR (having an fluorine ring) fiber shows a
slight enhancement of large signal gain in the C-band region. Gain
shapes in the L-band are virtually identical.
Example 3
L-band Fiber With and Without Fluorine Reservoir
Fibers suitable for L-band use were fabricated as in examples 1 and
2. Both fibers had the same nominal dopant and modifier cation
concentrations. Data on the preforms and fiber are shown below.
TABLE 2 Comparison of Fibers in Example 3 Fcore (fluorine Fring
(fluorine in the core of in the ring of Mfd (mode the preform the
preform field diameter Bkgd. Loss Fiber type drop) drop) of fiber)
Cutoff at 1160 nm Control .about.0.3 mol % N.A. 5.2 .mu.m 922 nm
13.7 dB/km DCLR >0.5 mol % .about.4 mol % 5.2 .mu.m 890 nm 5.9
dB/km
Example 4
Comparison of Effect of Thermal Processing on Directly Doped vs
Fluorine Reservoir Design Fiber
The present invention also provides a method to tailor radially the
fluorine distribution. In the present invention we provide a radial
distribution of the coefficient of thermal expansion (CTE) and
viscosity via diffusion of fluorine into the core from a region
outside the core.
The diffusion equation can be solved for the case of diffusion from
a distributed source in cylindrical coordinates. The radial
coordinate is r, the time is t and the concentration profile is
c(r,Dt). The initial concentration, c.sub.0, is distributed over
the shell from radius r.sub.1 to r.sub.2. The diffusivity, D, is
assumed independent of concentration. A derivation of this equation
may be found in Conduction of Heat in Solids, by Carslaw and
Jaeger, 1948. ##EQU1##
Example 5
FiberCAD Calculations on Depressed Clad No Ring and DCLR
Designs
With modeling software, such as Fiber_CAD from OPTIWAVE CORPORATION
in Ottawa, Canada, using as input preform profiles scaled to fiber
dimensions, the optical properties of fibers from two preforms were
calculated. The first fiber preform is an ebium-doped depressed
well profile. The second is an erbium-doped depressed well with a
fluorine ring (DCLR)
Core Calculated diam- Fundamen- eter Measured Calculated Measured
Calculated tal Mode (um) MFD (um) MFD (um) cutoff (nm) cutoff (nm)
Cutoff (nm) 3.21 5.21 5.24 919 780 1837 3.46 5.3 5.3 919 790
1804
The Peterman II mode field diameter is predicted well, but the
cutoff wavelength for the LP(1, 1) mode is not. Because of the
depressed well design of these fibers, a fundamental mode cutoff
occurs and the calculated values are given above. Because of the
deeper well of the fluorine pass, a slightly shorter cutoff is
predicted for fiber from the fluorine ring preform. The
calculations show that a DCLR design does not significantly alter
the mode field diameter of the fiber in the operating wavelength
range.
Those skilled in the art will appreciate that the present invention
may be used in a variety of optical article designs. While the
present invention has been described with a reference to exemplary
preferred embodiments, the invention may be embodied in other
specific forms without departing from the spirit of the invention.
Accordingly, it should be understood that the embodiments described
and illustrated herein are only exemplary and should not be
considered as limiting the scope of the present invention. Other
variations and modifications may be made in accordance with the
spirit and scope of the present invention.
* * * * *